The present invention relates to a hydraulic dual-circuit brake system for a road vehicle having front-axle and rear-axle brake circuits and, more particularly, to a system having, an electronic control device which emits a control signal for a valve arrangement such that a brake circuit which is, due to malfunction or the like causing deviation of the piston position from a setpoint range, is blocked off from an associated master brake line.
A brake system of the above-mentioned kind is shown in DE-37 23 916 A 1 in connection with a road vehicle which is also equipped with an anti-locking system. The wheel brakes can be blocked off individually or together from the master brake line of the respective brake circuit by driving solenoid valves, such as brake-pressure control valves, using output signals of an electronic control device. The brake booster provided in the known bake system is a twin master cylinder having a housing with two parallel cylinder bores which are arranged at a lateral spacing from one another and in which, in each case one master cylinder piston is bounded displaceably in pressure-tight fashion. The pistons form the axially movable boundary of a brake-booster output pressure chamber allocated to the front-axle brake circuit and the rear-axle brake circuit, respectively.
The actuating force, controllable by actuation of a brake pedal in the known system and, if required, increased by a brake power assist unit, acts on a plunger rod, which is axially displaceable on the housing of the brake booster in the direction of displacement of the master cylinder pistons and, to the master-cylinder end of which is swivellably coupled a two-armed rocker. The rocker arms of the rocker are supported axially, e.g. in each case via a tappet, against the master cylinder pistons, the actuating force thereby being "distributed" to the master cylinder pistons. The ratio L1/L2 of the lengths L1 and L2 of the rocker arms measured between the swivelling axis of the rocker and the central longitudinal axes of the master cylinder bores and their pistons corresponds to the ratio F1/F2 of the cross-sectional areas of the two master cylinder bores. The rocker with the shorter rocker arm is supported against the piston with the larger cross-sectional area. In this known construction of the brake booster, equal output pressures in the output pressure chambers of the brake booster also correspond to equal piston paths--assuming an equal degree of bleeding of the two brake circuits.
An electronic displacement sensor is allocated to each of the two pistons. The output signal of the sensor is a measure of the piston position and increases or decreases, for example linearly, with displacements of the pistons in accordance with brake pressure increase or reduction, respectively. The displacement sensors are designed in such a way that they emit signals with the same level for the same piston strokes. If, in the case of normal braking, i.e. braking not subjected to regulation, the output signals of the two displacement sensors signal that one of the two master cylinder pistons has been displaced by more than a threshold value, .DELTA.Smax, further in the direction of a brake pressure build-up than the other master cylinder piston, the electronic control device rates this as a poor degree of bleeding or failure of that brake circuit whose master cylinder piston has been displaced and produces an output signal or an output signal combination which moves the brake pressure control valve(s) of this faulty brake circuit into the blocking position. A warning signal which indicates to the driver that the brake system is faulty also flashes.
The blocking off of the wheel brakes of the faulty brake circuit from that output pressure chamber of the brake booster which is allocated to it prevents an excessive collapse of the brake pedal, which could irritate or surprise the driver, who is however nevertheless informed by the warning system. In the known brake system, the brake pressure control valve(s) of the faulty brake circuit remain closed until braking, detectable from the decay of the output signal of the brake light switch, is complete.
A disadvantage of the known brake system is the fact that if the blocked brake circuit is merely poorly bled, it can no longer contribute to the development of braking force in the case of a further increase of the actuating force and this can be associated with a considerable loss of otherwise achievable vehicle retardation. This is particularly serious when the faulty brake circuit is the front-axle brake circuit, via which the portion of the usable braking force is normally produced.
It is, therefore, an object of the present invention to improve a brake system such that, irrespective of an effective collapse limitation of the brake pedal when the front-axle brake circuit has failed, a maximum possible utilization of the brake force producible via the front-axle brake circuit is guaranteed in the case of only partial failure of the latter due to poor bleeding.
This object has been achieved in accordance with the present invention by providing the electronic control device with a comparator which compares the positions, represented by two output signals of two positions sensors, of the pistons of the brake booster to one another and produces a signal for the purpose of driving the valve arrangement into its blocking position as soon as the position of the brake booster piston allocated to the front-axle brake circuit corresponds to a stroke which is greater by more than a threshold valve, .DELTA.Smax, than the stroke of the brake booster piston represented by the position-indicating signal of the brake circuit and allows this signal to decay again when the difference of the piston strokes falls below a smaller threshold value, .DELTA.Smin. By virtue of the reconnection, now possible according to the present invention, of the wheel brakes of the faulty brake circuit to their allocated output pressure chamber of the brake booster as soon as the stroke difference of the brake booster pistons has decreased to a lower difference threshold value, Smin, the brake pressure in the poorly bled, brake circuit can be increased at least approximately to that amount which, given the poor degree of bleeding, can still be developed and a clear gain in terms of safety in the sense of stable braking behavior is consequently achieved in most practical cases. In the event of a failure of the front-axle brake circuit, the "pedal feel" achieved does at least not differ significantly from the pedal feel established in the case of braking when the brake system is completely intact and therefore also dose not mislead the driver into incorrect panicky reactions.
By virtue of a second comparator having a function which produces an output signal for driving the valve arrangement into its blocking position when the piston movably delimiting the front-axle pressure chamber reaches its end position, it is achieved that, as the brake pressure in the malfunctioning front-axle brake circuit cannot be increased further, that brake circuit is blocked off from its allocated output pressure chamber, after which a further brake pressure increase takes place only in the still intact brake circuit. Also, the pedal travel required to increase the development of braking force in minimized.
It is the purpose of a delay stage with a function to switch over the value arrangement into its normal throughflow position for a defined period of time that, as seen in relation to the functional condition of the brake system, an optimum front-axle/rear-axle distribution of braking force can be established in the course of a prolonged braking.
Switching over of the valve arrangement to meet requirements, with, as is were, variable "clock frequency", is obtained by a storage element which, when the position difference .DELTA.S of the two brake booster pistons is greater than a threshold value, .DELTA.Smax, is set to emit a signal effecting driving if the valve arrangement into its blocking positions and is reset by a comparator output signal.
As an alternative to the foregoing or even in combination with it, it is within the scope of the present invention to control the switching over of the solenoid valve arrangement with a fixed clock frequency by its own clock generator, with as high a clock frequency as possible being advantageous here.
Simple logic combinations of the comparator output signals and of an output signal of the delay stage which effect a driving of the solenoid valve arrangement suitable to the situation are used.
If a twin master cylinder having pistons activated via a moment-compensating rocker is provided as the brake booster, a particularly advantageous construction of the displacement sensors is possible in the form of inductive or resistive sensor elements.
Alternatively, brake booster pistons having a conical or ramp-shaped surface against which a mechanical feeler element can be radially supported are particularly suitable for a brake booster in the form of a tandem master cylinder.
If the vehicle is equipped with an anti-locking system, its brake pressure control valves can be used for the clocked blocking off of the poorly bled brake circuit.